Represented diagrammatically in FIG. 1a is a portion of an input video signal VIDEO_IN, corresponding to the displaying of an image line on the screen. This portion lies between two line-blanking intervals 11 and 12 designed for the flyback of the beam deflection armatures of the cathode ray tube. For the sake of simplicity, the video signal is considered to exhibit the form of a rectangular voltage pulse 13 inside the video signal portion considered.
Represented in FIG. 1b is an image improvement control signal PBC. This signal is synchronized with the signal VIDEO_IN. For example, the signal PBC is at the high level when the image is to be improved, and at the low level otherwise. In the example shown in the figure, the image is thus improved within a zone situated substantially at the centre of the line which corresponds to the portion of the signal VIDEO_IN between the intervals 11 and 12. This zone corresponds to a rectangular pulse 14 of the signal PBC.
Represented in FIG. 1c is a signal VIDEO_OUT at the output of a circuit which receives the signal VIDEO_IN as input, and which implements the image improvement functions. In the example considered here, this circuit is for example the screen's video preamplifier.
Outside the improved zone of the image, the signal VIDEO_OUT corresponds to the signal VIDEO_IN. Inside this zone, however, a pulse 13′ corresponding in the signal VIDEO_OUT to the pulse 13 in the signal VIDEO_IN differs from said pulse through the following characteristics:
the low level of the pulse 13′ is higher than that of the pulse 13, this resulting from the image brightness improvement function;
the difference between the high level and the low level of the pulse 13′ is higher than for the pulse 13, this resulting from the image contrast improvement function;
the pulse 13′ exhibits a positive peak 15 (an overvoltage) subsequent to its rising edge and a negative peak 16 subsequent to its falling edge, which result from the image sharpness (vivacity) improvement function. In the example represented, these peaks have exponential decay; and
the positive peak 16 of the pulse 13′ is clipped at the minimum level of the signal VIDEO_IN, this resulting from the black level clipping function.
The positive peak 15 participates in the sought-after sharpness improvement, which is dependent both on the amplitude and on the time constant of the peak 15. This is why it is also referred to as the “sharpness peak” in the jargon of the person skilled in the art and in what follows. The negative peak 16 also participates in the sharpness improvement. As is known in the state of the art, the negative peak 16 is nevertheless clipped at the black level, which corresponds to a zero value of the active component of the signal VIDEO_IN, so as not to disturb the operation of the display system.
In the state of the art, various ways of carrying out the sharpness improvement function are known. Apart from the entirely digital methods, methods involving differentiation of the input video signal and methods using a delay line are thus known.
The time charts of FIGS. 2a to 2c illustrate the principle of the methods involving differentiation of the signal VIDEO_IN. For simplicity, the signal VIDEO_IN (FIG. 2a) considered here is a rectangular pulse. By differentiation, one firstly obtains the differentiated signal VIDEO_IN (FIG. 2b), which exhibits a positive peak and a negative peak subsequent respectively to the rising edge and to the falling edge of the signal VIDEO_IN. Next by adding the signal VIDEO_IN and the differentiated signal VIDEO_IN, one obtains the output video signal VIDEO_OUT (FIG. 2c).
The time charts of FIGS. 3a to 3d illustrate the principle of the methods using a delay line. For simplicity, the signal VIDEO_IN (FIG. 3a) considered here is also a rectangular pulse. By passing the signal VIDEO_IN through a delay line, one firstly obtains the delayed signal VIDEO_IN (FIG. 3b). Next by differencing the signal VIDEO_IN and the delayed signal VIDEO_IN, one obtains a signal (FIG. 3c) comprising a positive peak and a negative peak subsequent respectively to the rising edge and to the falling edge of the signal VIDEO_IN. Then by adding this signal to the signal VIDEO_IN, one obtains the signal VIDEO_OUT (FIG. 3d).
However, the known implementations of these techniques by differentiation or by delay lines are not satisfactory in that they do not allow easy adjustment of the amplitude and of the time constant of the sharpness peak, and/or do not allow black level clipping, and/or are expensive to implement (in particular in the case of delay line techniques).